backend: avoid excception-hell in check_build_success()
    
    Previously, we expected that Builder.check_build_success() raised
    BuilderError in case of error, but this never happened - we instead
    raised RemoteCmdError which was never catched.
    
    Instead of cathing RemoteCmdError, execute `conn.run()` directly;  it is
    both faster (we ignore stdout/stderr) and both we can avoid the
    exception handling mesh in every call.
    
    The only exception which could be raised there is SSHConnectionError,
    which is carefully catched in worker.py.
    
    Relates: #987